Stroke is a neurological disorder which involves an acute injury to the cells of the brain as a result of the interruption of blood flow to the brain. When the blood supply to the brain is interrupted, the result is neuronal dysfunction and neuronal death or infarction. This interruption in the blood supply to the brain which is often referred to as ischemia, may result from a variety of causes, including an intrinsic blockage or occlusion of the blood vessel itself, a remotely originated source of occlusion, decreased perfusion pressure or increased blood viscosity resulting in inadequate cerebral blood flow, or a ruptured blood vessel in the subarachnoid space or intracerebral tissue.
Cerebral ischemia may result in either transient or permanent deficits. A transient ischemic attack (TIA) is a temporary neurologic deficit which clears in less than 24 hours. TIAs generally last less than 30 minutes. Symptoms of TIA include numbness or weakness of face or limbs, loss of the ability to speak clearly and/or to understand the speech of others, a loss of vision or dimness of vision, and a feeling of dizziness. A stroke is defined as a loss of neurons that usually results in a neurologic deficit that may improve but does not entirely resolve. The seriousness of the neurological damage in a patient who has experienced cerebral ischemia depends on the intensity and duration of the ischemic event. Ischemic events may involve single or multiple brain regions.
The likelihood of a patient having a subsequent stroke after experiencing a TIA or stroke is much greater than that likelihood in an individual who has not had a TIA or stroke. As a result of the higher likelihood of experiencing a stroke, doctors often provide medical treatments to patients who have had TIAs or strokes in order to prevent the development of a subsequent stroke.
Three types of stroke disorders have been identified, based on the underlying mechanisms involved in causing the ischemia, including thrombosis, embolism, and hemorrhage. Because it is often difficult to discern whether a stroke is caused by thrombosis or embolism, the term "thromboembolism" is used to cover both of these types of stroke. The term thromboembolism will be used throughout this patent application to describe thrombotic and embolic strokes. Determining the mechanism causing ischemia in a particular patient is an important step in the determination of an appropriate therapeutic profile for treating the ischemic disorder or for preventing subsequent strokes. Thromboembolic stroke is due to the occlusion of an extracranial or intracranial blood vessel by a thrombus or embolus. An embolus refers to any clot, debris or plaque which embolizes.
Hemorrhagic stroke, unlike thromboembolic stroke, is caused by the rupture of a vessel in the subarachnoid space or intracerebral tissue. This necessitates using different drugs for treating hemorrhagic stroke than for treating thromboembolic stroke. Most treatment methods for thromboembolic stroke involve the administration of drugs which can lyse the clot or prevent formation of subsequent clots. However, most drugs that lyse or prevent formation of blood clots also exacerbate bleeding. As a result, such drugs, which are useful in thromboembolic stroke, are contraindicated in treating hemorrhagic stroke because these compounds would induce further hemorrhaging (bleeding).
Antiplatelet agents, which inhibit platelet aggregation, are often used to prevent thromboembolic stroke in patients who have experienced a TIA or stroke. However, these agents only reduce the risk of subsequent attacks by approximately 25%. The most commonly used antiplatelet agents to prevent stroke include aspirin, ticlopidine and dipyridamole.
Anticoagulation agents also are used for preventing thromboembolic stroke. Anticoagulants, which prevent the coagulation of blood components and thus prevent clot formation, are used much more cautiously than antiplatelet agents. Anticoagulants are so effective at inhibiting blood clot formation that they may encourage or result in hemorrhage in the brain, causing hemorrhagic stroke. The most common anticoagulation agents used to prevent subsequent strokes are coumarin and heparin.
Although both antiplatelet agents and anticoagulation agents are effective in preventing subsequent cerebral ischemic events in patients who have previously experienced TIA or a completed stroke, thrombolytic agents are the therapy of choice during the first few hours after stroke onset. Thrombolytic agents function by lysing the clot which causes the thromboembolic stroke. Commonly used thrombolytic agents include streptokinase and tissue plasminogen activator (tPA).
Native t-PA is composed of 5 functionally distinct domains. These domains include the finger (amino acids 1-44), growth factor (amino acids 45-91), Kringle-1 (amino acids 92-173), Kringle-2 (amino acids 180-261), and serine protease (amino acids 264-527) domains. Deletion of the finger and growth factor domains results in t-PA having a lower binding affinity for fibrin than it would in the absence of deletion, suggesting that these domains may play a role in the binding of t-PA to fibrin. Zonneveld, A J et al., PNAS 83, 4670-4677 (1986); Verheijen, J. H. et al., Embo J. 5,3525-30 (1986)!. Although these studies also initially showed the Kringle 2 domain to have a potential role in fibrin binding, more recent studies involving mutations, suggest that the Kringle-2 domain may not be involved in fibrin binding but rather may be responsible for t-PA binding to lysine. Bennet W. F. et al. J. Biol. Chem. 266, 5191-5201 (1991)!. The serine protease domain which is believed to be responsible for the enzymatic activity of t-PA, has been shown to be involved in both the determination of fibrin specificity as well as fibrin binding. Bennet et al., 1991, Supra)!.
T-PA functions as a serine protease which converts plasminogen to plasmin in the presence of fibrin. Fibrin is believed to be a critical component of the protease activity because t-PA does not catalyze this conversion efficiently in the absence of fibrin. Sobel, B. E. et al., has hypothesized that it is this fibrin specificity of t-PA which allows t-PA to induce a local affect without causing systemic induction of plasminogen activation. Circulation 69, 983-990 (1984)!. The phenomenon of local activation is unique to t-PA as opposed to other non-fibrin-specific thrombolytic agents, such as streptokinase and urokinase, which tend to induce a higher systemic plasminogen activation.
Native t-PA generally exists in either a single chain or a double chain form, both of which are functionally active. The single chain form of t-PA includes all 527 amino acids. The double chain form is produced by cleavage of the single chain form between amino acid positions 275 and 276. Although both the single and double chain forms are functionally active, each of the different forms of t-PA exhibit a different level of enzymatic activity in the absence of fibrin. The double chain form exhibits greater enzymatic activity than does the single chain form. In the presence of fibrin however, both the single and double chain forms of t-PA are equally active. Lijnen et al., Thromb. Haemost. 64, 61-8 (1990), and Bennett et al., 1991, Supra)!.
Acute stroke is an urgent medical condition with a small therapeutic window of time (possibly as brief as 4-6 hours) in which treatment is beneficial. Clot lysing drugs are believed most useful if administered during this window of time in order to at least partially restore cerebral blood flow within the compromised region and to sustain neuronal viability.
Recently, investigators have begun to study the effect of thrombolytic agents as a treatment for acute thromboembolic stroke during this critical window of time. The studies have revealed that tPA is effective in improving neurological outcome in patients who are treated within the first few hours of onset of the stroke. A significant clinical improvement was seen in patients who were administered tPA within 90 minutes of stroke onset. These findings indicate that tPA may be an effective clinical treatment method for acute thromboembolic stroke. Although tPA appears to be both safe and efficacious, there is a need to improve both the rate and speed of restoration of blood supply to the brain.
In an effort to improve treatment for acute stroke, researchers set out to determine whether combining thrombolytic therapy with other therapies would improve the physiological outcome in stroke patients. A study by Thomas et al., Stroke V. 26, page 1039-1046 (1995) looked at the effect of combining aspirin, an antiplatelet agent, with tPA, a thrombolytic agent, in a rabbit model of thromboembolic stroke. Although it has been known in the art that this combination of thrombolytic therapy with antiplatelet therapy in acute myocardial infarction has been beneficial to the patient and reduced the death rate by greater than 50% in myocardial infarction patients, the authors in the Thomas study found that aspirin actually antagonized the effect of tPA in thromboembolic stroke. Because aspirin is often used for preventing stroke in patients who have previously suffered a stroke or a TIA, the finding that aspirin actually antagonizes tPA induced clot lysis and exacerbates brain injury and acute stroke may have devastating results in these patients. The findings suggest that tPA should not be used in combination with antiplatelet agents.